Various solder fluxes are used in conjunction with soldering materials in the soldering of electronic components, circuits, equipment and the like so as to improve the efficiency and quality of the soldering operation and to improve the long-term reliability of the connections. Solder fluxes are often designed to react with or dissolve metal oxides and impurities on the surfaces being soldered, and to coat the surface to protect it against oxidation.
The use of flux in soldering operations that involve items having very small features, as in ball attach operations on wafers for Wafer Level Chip Scale Packaging (WL-CSP), has placed great demands both on the flux itself and on its method of application. Because of their dimensions, devices of this type have very small tolerances for error in terms of the placement of a solder joint. Consequently, if the solder migrates from its original intended position on the substrate during reflow, electrical bridging can occur between neighboring solder joints, leading to a defective product.
One cause of solder migration is the flux itself. If the flux is not sufficiently aggressive, the solder can migrate across the surface of the flux during reflow, thus leading to the bridging problems noted above. In the past, this problem has been addressed through the selective application of solder flux to a surface through the use of a stencil or template. In theory, a stencil can minimize solder migration by limiting the area to which the solder flux is applied, hence limiting the area over which the solder can migrate.
FIGS. 1–3 illustrate the use of a stencil in applying a solder flux. In this approach, as shown in FIG. 1, a stencil 11 is applied to a wafer substrate 13. The stencil is equipped with a plurality of apertures 15 adapted to receive a solder flux. These apertures are spaced to direct the flux over the under bump metallization layer 14. The under bump metallization layer is in turn disposed on the bond pad 16. The solder flux 17 is then applied across the template using a squeegee 19. As shown in FIG. 2, after the solder flux 17 has been applied, the stencil 11 is removed, with the effect that the solder flux is applied only in the vicinity of the apertures. As shown in FIG. 3, the solder balls 21 are then positioned onto the solder flux 17 and are reflowed.
The stencil approach described above is undesirable in that the use of a stencil inherently complicates the manufacturing process. Also, any misalignment between the stencil and the substrate will result in the flux being improperly applied, thus resulting in product defects. Because of the dimensions involved, however, alignment is very difficult to control. Moreover, while the proper use of a stencil may be adequate in ensuring that solder flux is applied only to intended areas of a wafer substrate, this approach cannot ensure that the solder flux will not subsequently move from those areas during solder ball placement or reflow, thus compromising the benefits of using the stencil in the first place.
There is thus a need in the art for a solder flux, and for a method of applying a solder flux to a substrate, that can be used for ball attach operations on Wafer Level Chip Scale Packaging (WL-CSP) as well as in other solder ball attach applications, that does not require the use of a stencil for its application, and that minimizes solder migration. These and other needs are met by the compositions and methodologies described herein.